Okay you can't tell too well from this picture but I cute a hole about an inch square in the bottle corner opposite the handle.
Then I cut off the top, being careful not to mess up the handle.
Here's a tip that I didn't know. Your plants should still be pretty small. I had a larger plant and had to slice up the side to get it to fit in then taped it over. I don't know how well this will work in the long run. But we're experimenting here. So shake off some of the dirt around the root ball.
Now after practice it's easier to insert it from the top then it was to try and squeeze the root ball through the small hole at the bottom. Don't laugh at me, I'm sure other's have done that too. Just carefully thread the plant through the hole.
I cut up an old cotton sweater to help brace the plant. Other instructions have used either nothing at all or those basket liners. I didn't want to just leave it bare, the sides of the milk carton seemed kind of sharp. Plus it helped keep the soil in.
Here they are all done, I still have to hand them outside but it still gets a little chilly in the evenings. In another week or two it should be good.